TOPIC: Buffer Gets per Row with Aggregates

In one of your white papers (and if memory serves, in your classes as well) you have a rule of thumb that an OLTP query should be tuned until buffer gets per row falls below 100 and preferably closer to 10. The footnote of your white paper says:

"All ratios are prone to fallacies, and this one is no exception. For queries that return aggregated data (sum, count, min, max, etc.), the acceptable limit of this ratio is of course much higher."

How do you arrive at an appropriate target for those queries that do return aggregated data?